Dried water-borne coatings can build up overtime inside the coating supply lines and outside on bell cups or robot caps of coating application equipment. The dried or aged water-based coating cannot be efficiently removed with water alone. If the dried coating is not removed from the application equipment, the potential for it to break free or chip off can cause a coating defect in the finish such as the finish of an automobile.
Current commercial practice utilize flushing solutions that contain amines and an organic solvent selected from lower alcohols and glycol ethers to remove residual coating material from surfaces of coating application equipment. Flushing solutions can be prepared on-site by adding water to flushing concentrates, which are typically marketed to the end user. Generally, the flushing concentrates contain 85% to 99% solvent and 1% to 15% alkanolamine. The flushing concentrates are usually diluted to 5% to 15% by volume, with deionized water. The diluted flushing solutions are then passed through the coating application equipment to remove any unwanted residual coating from surfaces of the equipment.
The alkanolamines that can be used in the solutions include monoethanolamine (MEA), dimethylethanolamine (DMEA), monoisopropanolamine (MIPA), diethanolamine (DEA), and mixtures of primary and secondary alkanolamines. Typical solvents include lower alcohols and glycol ethers such as ethylene glycol n-butyl ether, diethylene glycol n-butyl ether, triethylene glycol methyl ether, propylene glycol n-butyl ether, dipropylene glycol methyl ether, propylene glycol methyl ether, and propylene glycol n-propyl ether. Unfortunately, there is not a single flushing solution that can be used or is ideal for all types of coatings. Different proportions and combinations of alkanolamines and solvents need to be determined for various types of coatings.
One problem that can result in removing a coating from surfaces of coating application equipment is that the flushing solution causes the coating to wrinkle or blister resulting in flakes or chips. These flakes or chips that do no eventually dissolve during the flushing process can remain in the equipment. Subsequent use of the equipment can dislodge the flakes and cause finish imperfections. As a result, it is not only desirable for the flushing solution to remove the coating in a reasonable amount of time, but to also remove the coating by minimizing blistering, and hence formation of flakes. Developing the right flushing composition to remove a specific type of coating, yet minimize flaking is often quite a challenge.
A second problem that arises in commercial settings is the cooling of flushing solutions as they sit or pass through the coating application equipment. Presently, some automotive assembly plants use rotating bell applicators to apply water-based coatings to the vehicle body. The bells spin at 20,000 to 60,000 RPM during the coating process. Shaping air is also blown over the bells to direct the coating to the vehicle. The bell cup and housing unit can also act as a heat sink. The rotation of the bells, shaping air, and the equipment itself all contribute to cooling previously heated flushing solutions to ambient or near ambient temperatures during a cleaning or purge cycle.
Cooled flushing solutions are generally not as effective in removing residual coatings. Due to different manufacturing protocols and differences in coating equipment some commercial manufacturers experience more cooling of the flushing solution than is desired. As a result, there is a need to develop flushing compositions that are effective at removing coatings at or near ambient temperatures.